In the GSM system connection reconfiguration concerns the modifying of a call mode. The procedure is known as in-call modification. The term ‘mode’ means the operational status of a call; it can be for instance a standard speech mode, data mode, fax mode, an alternating speech/data mode or an alternating speech/fax mode. When a connection is reconfigured, its mode can thus be changed e.g. from a speech mode to a data mode. In case the channel used for the connection does not support the required characteristics, channel configuration can be changed. The solution known from the GSM system is not, however, applicable for use in UMTS (Universal Mobile Telephone System) described below. The reason for this is that in the UMTS a single connection can simultaneously use one or more radio bearers. The characteristics of the radio bearers may have to be modified upon establishment of or during a connection.
The term ‘radio bearer’ refers to a service provided by a network layer. Multimedia service typically uses a plural number of radio bearers simultaneously for providing a service. Video telephony, for example, may require four different radio bearers: transmission of speech and image both use separate radio bearers for uplink and downlink. A multimedia service, such as video telephony, can also be implemented by using only one radio bearer per transmission direction, thereby avoiding the problem of synchronization between radio bearers of the same transmission direction. Radio bearer parameters comprise most of the first and second layer operational parameters. A radio bearer user, however, does not know the parameters of lower layers. Therefore the radio bearer user is not aware of how the radio bearer provides its services, i.e. whether it uses a half of a TDMA time slot, one time slot or a plural number of them, or one or more CDMA spreading codes.
A radio bearer is defined by a set of parameters or attributes that concern the traffic or quality characteristics of a service provided. A radio bearer is not to be considered similar to a logical channel, which is a service provided by a data link layer.